The frog Xenopus laevis contains two forms of the ets-2 gene (ets-2a and ets-2b), each encoding proteins of 472 amino acids, a number close to that of the human, mouse, and chicken homologs. The percent identity between the Xenopus ets-2a and ets-2b products is 94.1, while that between ets-2a and the other vertebrates is about 68%. The Xenopus sequences are homologous to the other vertebrate ets-2 genes throughout their entire length. Because of the ease of manipulating oocytes and embryos, the Xenopus system should provide a useful means of studying ets-2 function, and results of studies performed with the Xenopus homolog should shed light on the function of this proto-oncogene in humans. A molecular clone containing sea urchin DNA that hybridized to a v-ets probe was shown to contain an exon of the erg gene. This segment codes for 173 amino acid residues beginning at a 5' splice site. The first 85 residues share homology with all members of the ets gene family (including the erg, elk, Drosophila E74, mouse PU.1 and ets-I and ets-2 genes), while the remaining 88 amino acids are homologous only to the human ERG gene. This latter region, designated R, represents a highly conserved erg-specific domain. Phylogenetic analysis of ets sequences indicates that the family may be divided into three major groups: (1) the ets genes proper including the vertebrate ets-I and ets-2 genes, and the sea urchin and Drosophila ets genes; (2) the human and sea urchin erg genes; (3) the human ELK, the murine PU.1, and Drosophila ecdysone-induced puff 74E genes. Since each of the groups contain representatives of both vertebrate and invertebrate phyla, it is possible to estimate that the gene duplication events from which they originated occurred at least 500 million years ago.